Hyperreactio luteinalis (HL) and spontaneous ovarian hyperstimulation syndrome (OHSS) are both rare conditions during pregnancy. The clinical presentation of HL and OHSS are comparable and both should be differentiated from ovarian carcinoma. We present a case of a 32-year-old woman who was initially seen with markedly enlarged multicystic ovaries and ascites in the 13th week of a spontaneously conceived pregnancy. Ultrasonographic follow-up and magnetic resonance imaging of the ovaries were employed in order to avoid exploratory laparotomy and rule out ovarian carcinoma. The patient received supportive therapy and delivered a healthy child at term. The increasing use of ultrasonography may lead to more frequent findings of multicystic ovaries in spontaneously conceived pregnancies. Making the distinction between HL and spontaneous OHSS in these cases may be difficult though clinically irrelevant as the approach to treatment is similar in both.